Fiber reinforced concrete

ABSTRACT

Fiber reinforced concrete has thin steel wire of diameter between 0.05 and 0.3 mm such as cut from recycled vehicle tires. To avoid the problem of balling when mixing, two alternatives are suggested. The first consists of strands of fiber, which demonstrate excellent bond characteristics. The second consists of a mixture of fiber lengths and thicknesses, giving a wide distribution of l/d ratios not exceeding 250, which has the effect of reducing balling tendency so that significant densities can be achieved.

The present invention is in the field of fiber reinforced concrete.

It is known to reinforce concrete using steel cages of welded rods, orindeed individual rods tied together. It is known that these kinds ofreinforcements present some problems, primarily because that type ofreinforcement is on a “macro” level. Where concrete is required to belocally tough, or the geometrical shape to be reinforced is complex,such reinforcement is not very effective.

It is known to reinforce concrete using fibers, often of steel. Fibersare effective in reinforcing concrete locally, preventing cracking andsurface deterioration, as well as providing structural reinforcement.

A problem with such fibers is that, if they are long and rigid (which,with steel, means having a length to diameter (l/d) ratio in excess ofabout 100, especially when volumes of fibers above 1% are used) then thefibers tend to ball together and prevent even mixing and distribution ofthem throughout the concrete. Indeed, the more they are mixed, the morethey ball together which, thereafter, prevents the concrete from beingpoured or pumped or cast, as is normally desirable with concrete.

It has been proposed to glue fibers together with water solubleadhesive. By such means, l/d ratios of the individual fibers of as muchas 80 can be used even for fiber volumes higher than 2%. This isachieved because, when the bundles of glued fibers are first introducedto the concrete mix, the bundles can be evenly distributed before themoisture in the cement and aggregate mix dissolves the adhesive. At thispoint the individual fibers separate from the bundle, but they needdistribution then only over a local space. Relatively even distributionof the entire stock of fibers is thereby achieved before balling canstart to occur.

However, even with this measure, performance is lacking in two keyareas.

The first is simple, and this is that if fiber densities approach orexceed 2% by volume, mixing problems become an issue. From the latterperspective, it is more usual not to exceed ½%. Therefore, thereinforcement capacity of the fibers is limited.

The second problem is more complex. To be effective, the fibers must beanchored in the concrete. This is so that strain in the concrete isimmediately shared by the reinforcement, “mobilising” the reinforcementto provide tensile support to the concrete to resist its cracking.

Steel presents a relatively “slippery” surface to concrete and, as ageneral rule, l/d ratios of the order of not less than 100, and ideallyabout 200 for high strength fibers, are needed to ensure completemobilisation of the reinforcement. But with l/d ratios not exceeding 50,or at best about 80, such mobilisation cannot fully occur. For similarl/d ratios, smaller diameter fibers are more effective in transmittingloads.

This problem is overcome to some extent by kinking the ends of fibers toform anchors (as disclosed in DE-A-4315270). This means that a crackdeveloping across a fiber, even relatively near one end of the fiber,will transmit load to the fiber. However, the fiber becomes essentiallyfree along its length (at relatively high stresses) within its sleeve ofsurrounding concrete because there is insufficient area of the fiber onwhich the concrete can bond. Consequently, the body of the fiber becomesunbonded and the stress is developed over the entire length of thefiber. This means that substantial strain must be imposed before thetension in the fiber balances, and counteracts, the stress in theconcrete. This, in turn, means that a developing crack will widen morebefore it is halted. More, that is, than if, for example, a much shorterfiber spanned the crack while still being anchored at either end: theextension of such a short fiber would be much less for the same stressthan a longer fiber.

Furthermore, using high tensile strength steel adds little benefitbecause the strength capacity of the fiber is substantiallyunder-utilised. At stresses at which such material would normally yield(that is, exploiting their full strength capability) the fiber wouldlong previously have pulled itself out, even with the anchoring providedby a kinked end.

Consequently, not only can insufficient quantity of reinforcement beemployed to provide adequate reinforcement (at least for moresignificant structural loads) but also the capacity of what is, or couldbe, employed cannot be fully exploited.

To address this problem many other solutions have been proposed,including modifying the surface of the fiber as in U.S. Pat. No.5,451,471, DE-A-4242150, U.S. Pat. No. 4,960,649, U.S. Pat. No.4,804,585, DE-A-3435850 or EP-A-105385, or modifying the cross sectionof the wire as in DE-A-1941223, U.S. Pat. No. 4,298,660, or even usingchains as JP1153563. All of these methods result in expensivereinforcement.

EP-A-861948 suggests thin, high tensile steel wire with anchoragesformed across and along its length; The thickness is about 0.08 to 0.3mm, and the length is from 3 to 30 mm. The tensile strength is about2000 MPa. Because of the high bonding and high strength, small volumesare adequate to achieve the desired reinforcement (1 to 4% by volume issuggested), which small volumes eliminate mixing problems, at least withl/d ratios below 100. DE-A-3347675 likewise suggests thin wires withsurface roughening to improve adhesion to the concrete. Both thesearrangements suffer from the expense of the special working of the wirerequired.

NL-A-7108533 suggests reinforcing concrete material with steel cord ofthe type used in car tires where the cord comprises several threadswound together with between 50 and 100 twists per meter.

BE-A-1003656 discloses packaging steel reinforcement fibers by gluingwith water soluble adhesive, or otherwise temporarily securing, the endsof the fibers to a paper carrier.

It is an object of the present invention therefore to provide a fiberreinforcement structure or composition for concrete that does not sufferfrom, or at least mitigates the effects of, the aforementioned problems.

In accordance with a first aspect of the present invention there isprovided a fiber reinforcement structure for concrete, comprising cleansteel fiber of between 0.05 and 0.3 mm diameter, wherein the fibers arestranded together in a strand (or cord) of at least five, and preferablyat least twenty, fibers, characterized in that the ends of the fibers inthe strand are permanently and structurally secured together, forexample by welding.

Preferably, each fiber has an l/d ratio in excess of 150and the strandan l/d ratio of less than 60. Preferably, the strands have a diameter ofabout 1.55 mm and a length of about 50 mm. There may be at least twentyfibers in each strand. Each strand may comprise an inner core of between10 and 15 fibers and an outer sleeve of 12 to 20 fibers. The strandpreferably comprises an inner core of 12 fibers, and an outer sleeve of15 fibers. Preferably, there is little or no twist of the majority ofthe fibers in the strand.

By “clean” is meant less than 5% by volume rubber or other contaminationof the fibers and sufficiently grease—and contamination—free to permitbonding of concrete cement to the fibers.

With such thin wire, and such a large l/d ratio, secure bonding of thefibers in the concrete can be assured. This means that cracks open lessbefore the stress is applied to the fiber which, over such a shortlength of it, tensions rapidly to balance the stress with only a smallstrain at the concrete crack. Consequently, the crack is not openedmuch, and so secondary effects such as environmental contaminant ormoisture entry are minimised. Since more secure bonding is achieved,higher stresses can be absorbed, thereby more efficiently utilising thefull strength capacity of the fiber.

Furthermore, the problem of balling or clumping with the high l/d ratiofibers is overcome by virtue of the stranding of the fibers. When mixingin concrete, the strand behaves as a single fiber having an effectivel/d ratio determined by the length and diameter of the strand. In atwenty-fiber strand, for example, this reduces an l/d ratio of 150 of asingle fiber to about 30 of the strand. However, because the cement,when hydrated, can penetrate all around the outside fibers and abouthalf way around each of the fibers underneath (ie effectively aboutfifteen out of twenty in a twenty-fiber strand), the net result is thatbonding to the strand is over a much greater surface area. It is up toan order of magnitude greater than bonding to a single fiber (ofequivalent l/d ratio of the strand as a whole—ie about 30).

Since the strand can, therefore, have an l/d ratio of as little as 30,clumping is not a problem and so the volume of the reinforcement can beincreased to as much as 2% by volume or more. Consequently, not only canmore reinforcement be provided, but what reinforcement there is is usedto greater efficiency because of the improved bonding of the concrete tothe strand.

Indeed, although the improved bonding of the outer fibers of the strandto the concrete causes more instant mobilisation of those fibers tominimise the strain in the concrete, the inner fibers are, to a certainextent, free. At least, they are free intermediate their ends but theyare, nevertheless subject to frictional constraint against theirneighbors. However, over and above such frictional constraint, shouldthe strain in the concrete develop such that outer fibers of the strandbegin to yield, reinforcement remains through the inner fibers whichhave their full length with which to absorb the strain.

Preferably, there is little or no twist of the majority of the fibers inthe strand. Indeed, there may be less than 100 twists of the fibers inthe strand per meter. This has the effect of maintaining the axialstiffness of the strand, but it also permits some lateral flexibility,which helps reduce the effect of balling and enables the strand to flexaround large aggregate. Preferably, the strands are made by cutting tolength cord or wire strands from recycled car and vehicle tires.Preferably said tires have been subject to pyrolysis or anaerobicmicrowave heating to strip elastomer from the wire strands, withoutdamage to, or leaving much residue left on, the steel. Preferably, saidtires have been subject to a process as described in WO-A-01/03473, theentire contents of which are hereby incorporated by reference.

Thus not only can effective reinforcement for concrete be provided, butalso it can be got from vast stocks of waste material in the form of oldtires. The raw material is therefore almost cost-free, the necessaryprocessing to remove rubber from the strands not causing the cost of thestrands to become excessive.

On the other hand, such processing of tires is not completely withoutcost, and it would be desirable to have fiber-reinforcement, perhaps toa lesser degree, using unwanted steel from tires without suchprocessing.

DE-A-4104929 discloses using wire from tires, but mixesrubber-bound-fiber mixed with non-flammable concrete components, therubber being burnt off prior to cooling and adding of cement and water.The rubber is left in place during mixing with concrete components toavoid balling problems. It does not appreciate that strands can have alow “macro” l/d ratio and still provide effective bonding to concrete.Consequently, they do not require the protection against ballingsuggested.

On the whole, tires are presently recycled to a certain extent byrepeated shredding, combing, magnetic separation and sifting to releaserubber granules which can be employed in numerous applications. However,the steel waste has hitherto defied efficient usage because of thecontamination with rubber and textile fiber and is generally baled anddeposited in landfill. Some less sensitive furnaces can use the waste asraw steel source, but it is not very cost-effective.

DE-A-3923971 describes a process for mechanically and cryogenicallystripping wire “nails” from tires for the purpose of recycling thenails. In the situation that the nails still have rubber connected, theyare used as a filler to improve elasticity of the filled material.

Accordingly, in an alternative aspect of the present invention, there;is provided a fiber reinforcement composition for concrete comprisingsteel fiber obtained by shredding vehicle tires and physicallyseparating therefrom non-steel material until “clean” wire fibersremain, about 90% or more of them being individual fibers andsubstantially none having an l/d ratio of more than 250, characterisedin that a majority of the fibers are less than about 0.5 mm in diameter,any wider diameter fibers having an l/d ratio less than 100.

It is also preferable that a majority of the fibers are about 0.3 mm orless in diameter and have an l/d ratio between 150 and 250. Betterstill, if more than 80% of the fibers are about 0.3 mm or less indiameter and have an l/d ratio between 150 and 250.

In this aspect, the quantity of fibers being referred to is theirnumber.

It is suggested above that an upper limit of 100 for the l/d ratio offibers is needed if balling when mixing sufficient amounts of fiber (i.e2% by volume) is to be avoided. However, when the flexibility of thefiber is high (as it is with steel wire of less and 0.3 mm diameter) itis found that balling can be avoided to a sufficient extent when mixingconcrete if the l/d ratio does not exceed 250, and especially if keptbelow 200. On the other hand, at these lengths, even though there may bea certain contamination that interferes with bonding where it occurs, nosurface preparation of the wire, such as suggested in EP-A-861948, isneeded to ensure adequate bonding. In addition, the wires of tireshaving been processed as described are far from straight, so that thereis inherent kinking of them, which assists locking of the fiber in theconcrete.

Consequently, the essence of the present invention in this second aspectis to avoid as much as possible long, wide-diameter, and thereforestiff, wires, but at the same time maximise long, thin diameter wires.This can be achieved through appropriate mechanical processing of thetires. Thus, the environmentally challenging methods employed inDE-A-4104929 are unnecessary, since essentially only thin wires' arepermitted to have longer l/d ratios that guarantee good bonding, butwhich do not cause balling problems to the same extent as thicker,stiffer wires of the same l/d ratio.

In order to achieve substantially no fibers of l/d ratio above 250,repeated shearing of the fibers is required. However, this has the sideeffect of also shearing shorter fibers so that even those below 250 mayalso be sheared. Consequently there needs to be separation of shorterfibers from the mix as they are produced and so that they are notchopped further. This is with the aim of achieving most fibers having anl/d ratio range of between 150 to 250. At the same time longer, wide(that is, stiff) fibers (for example, fibers greater than 0.3 mm indiameter) need to be removed so that they, on the whole do not have anl/d ratio greater than 100.

“Clean” as used in this aspect of the invention has the same meaning asthat given to it above. Wire fibers, resulting from such a shreddingprocess, surprisingly provide an effective concrete reinforcementstructure. Firstly, despite not providing perfectly clean fibers, aphysical shredding process is found to be adequate to achieve sufficientbonding between the fibers and concrete cement, particularly given thel/d ratios suggested. On the other hand, the invention does notspecifically exclude further treatment to remove more contamination.Secondly, although the invention requires a minimum quantity of high l/dratio fiber, it is, in fact, this quantity that determines, and limits,the mixability of the composition or structure. With such long fibers,balling becomes an issue the more long fibers there are. However, it isnot found that any less long fibers can be introduced, merely becausethere is also a proportion of shorter fibers introduced as well. Whilevery short fibers do little to enhance the quality of concrete, shortfibers more than a few millimeters long do enhance concrete toughnessand wear resistance.

Consequently, it is found that the density of steel that can be added toa concrete mix can be quite reasonable, and in the order of 1 to 4½% byvolume of the final mix. Thus, tough fiber-reinforced concrete can bemade wherein the reinforcement is used to its maximum extent. That is tosay, the long fibers provide strength to the concrete, being highlyresistant to pull out under tensile load. On the other hand, the shorterfibers, while not detracting at all from the strength, provide,nevertheless, a substantial part of the toughness of the concrete andits resistance to wear. That toughness is also provided by the longfibers, of course, but the contribution made by the shorter fibers is noless important in this respect. By shorter fibers is meant those havingan l/d ratio less than about 150. Thus the long and short fibers eachperform complimentary roles in reinforcing concrete, the whole of thereinforcement being greater than merely the sum of their respectivecontributions.

Such a distribution of wire fibers can be generated by, indeed, is to acertain extent a natural consequence of, repeated shredding and shearingof car or other vehicle tires, and with subsequent magnetic extractionof the wire from the remaining fabric and elastomer. However, care hasto be taken that, in shredding and shearing to remove fibers of greaterthan 250 in l/d ratio, excessive cutting of fibers less than 250 in l/dratio is minimised. It is desired that the proportion of fibers havingl/d ratios in the range 150 to 250 is maximised. At the same time,thicker wires (ie greater than about 0.5 mm in diameter), even thosewith a large l/d ratio, are most desirably removed and limited to thosewith no more than about 100 l/d ratio. Indeed, the shorter that thickwires become, the less effective they are as reinforcement, andconsequently their entire removal from the composition is preferred.

It is a feature of both aspects of the present invention that theyprovide outlets for the recycling of vehicle tires, and an inexpensivesource of effective reinforcement for concrete.

An important element of the second aspect of the present invention isthe mix of the concrete. That is to say, the size distribution andmake-up of the aggregate, as well as the type of cement, all have animpact on the tendency of the fiber element to ball when it is mixed.Generally, an increase in fines reduces balling, but it remains thatsome trial and error might be required to find satisfactory mixes thatachieve the aims of the present invention, at least in its secondaspect.

In both aspects of the invention, the fibers could be used to produce(a) SIMCON (Slurry Infiltrated Mat Concrete), (b) SIFCON (SlurryInfiltrated Fiber Concrete), and (c) high-strength, high performanceconcrete.

SIMCON is particularly suited to the second aspect of the presentinvention, since a very thin mat (similar to glass fiber chopped strandmat) can be used to create thin structural elements of thickness notexceeding a few millimeters. SIMCON is also suitable for near surfacereinforcement of thicker elements. The thin mat of fibers can beproduced preferably by using polymer adhesives or welding or stitchingof the steel fibers.

SIFCON can be produced with both aspects of the present invention, in amuch more economic way than with current systems, especially whenrecycled fibers from tires are used.

For high strength high performance concrete, both aspects can be usedsimultaneously.

Embodiments of the invention are described hereinafter, by way ofexample and with reference to the following examples, in which types ofconcrete are prepared as follows:

EXAMPLE I A Typical Normal Concrete

Total Weight 100 Ordinary Portland Cement 16.5 Water 7.5 Fine Aggregate30 Coarse Aggregate (Crushed 46 River Aggregate <20 mm) Water/Cementratio 0.45

EXAMPLE II A High Strength Concrete

Total Weight 100 Ordinary Portland Cement 15.4 Type of pulverised fuelash 4.4 Micro-silica 4.4 Water 3.5 Fine Aggregate 29.7 Coarse Aggregate(Crushed 42.6 River Aggregate <20 mm) Superplasticizer 1.5% (Weight ofCement) Water/Cement ratio 0.23

Materials Used

Ordinary Portland Cement

An ordinary portland cement (OPC) type I, manufactured by Rugby CementGroup: in accordance with BS 12: 1996, class 42.5N, was used through thestudy. The typical chemical and physical properties of the cement aregiven in Table 1.

Table 1. Chemical and physical properties of the OPC used

TABLE 1 Chemical and physical properties of the OPC used ChemicalComposition Percentage Physical Properties Silica SiO₂ 20.5–21.8Relative density: 3.1 Alumina AlO₃ 5.1 Theoretical surface area: 3800m²/kg Iron Fe₂O₃ 3.7 pH in water N/A Calcium CaO 64.6 Moisture contentN/A Magnesium MgO 1.3 Comp strength EN 196-1 Mortar Prisms Sulphate SO₃2.3–3.1  3 days 20.6–21.6 N/mm² Alkalis 0.57–0.74  7 days 34.8 N/mm²Chlorides Cl⁻ <0.02 28 days 42.6–43.4 N/mm²Aggregates

The aggregate used (both coarse and fine) was fluvial dragged gravel.The shape of the aggregate was rounded, fully water-worn or completelyshaped by attrition, i.e. river or seashore gravel; desert, seashore andwind-blown sand. The surface texture was smooth, water-worn, or smoothdue to fracture of laminated or fine-grained rock, i.e. gravels, chert,slate, marble, some rhyolites. These classifications are made accordingto BS 812: Part 1:1975. The aggregate grading was made according to theBS 812: Part 1:1975, the results of this grading are shown in the Table2, and Table 3. Other properties are given in Table 4.

Table 2. Coarse aggregate grading

TABLE 2 Coarse aggregate grading Sieve size 20 mm aggregate 10 mmaggregate (mm) Passing (%) Passing (%) 37.5 100 100 20 98 100 14 57 10010 12 95 5 05 7 2.36 — 0.65

Table 3. Fine aggregate grading

TABLE 3 Fine aggregate grading Sieve size Fine aggregate sands (mm)Passing (%) 9.5 100 4.75 98 2.36 88 1.18 80 0.6 68 0.3 23 0.15 4 0.0750.5

Table 4. Other Material Data

TABLE 4 Other Material Data Water Water Density Absorption Content OPC3150 Sand 2590 0.59 0.10 C. Agg. (20) 2600 0.58 0.09 C. Agg. (10) 26000.60 0.34Steel FibersSteel Stranded Wires (First Aspect)

The stranded wires used were obtained from the process described inWO-A-01/03473 (“the AMAT process”). The wire was derived primarily fromsuper-single tires. The wires used had an overall average diameter of1.38 mm. The wire-consisted of an inner core of 12 strands of diameter0.22 mm, an outer sleeve of another 15 wires of diameter 0.22 mm, and anoverwound wire of diameter 0.22 mm at a pitch of 5.33 mm. The wires hadtraces of carbon black on the surface.

Fibers from Shredded Tyres (Second Aspect)

The fibers used to make the concrete of the second aspect of the presentinvention were obtained from a shredding process, dealing primarily witha mixture of truck tires. The fibers were not completely free of rubber,having around 3% rubber by weight. The fibers used had the propertiesdescribed below with reference to FIGS. 8 to 10 in terms of their length(L), thickness (D) and l/d ratio. The strength of the fibers varied from2000 MPa to 3000 MPa.

The invention is further described hereinafter, by way of example, with,reference to the accompanying drawings, in which:

FIGS. 1 a and b are photos of stranded wire derived from the AMATprocess, in FIG. 1 a, the strands being separated into their individualfibers, whereas in FIG. 1 b the strands are intact;

FIG. 2 shows fibers from shredded tires prior to further cleaning andsorting;

FIG. 3 is a photo of a concrete sample according to Example I abovedemonstrating adequate workability;

FIG. 4 is a graph showing deflection of a concrete sample according toExample I above with, and without, shredded fibers of the second aspectof the present invention;

FIG. 5 is a similar graph comparing the first and second aspects of thepresent invention, in concrete from Example I, and also comparing withthe same concrete employing presently available commercial fibers;

FIG. 6 compares normal concrete with no fibers, normal concrete withtire strands according to the first aspect, and high strength concreteof Example II, with tire strands from the first aspect of the presentinvention;

FIG. 7 compares increasing density of tire strands in concrete ofExample I;

FIG. 8 shows the length distribution of fibers from shredded tires(second aspect);

FIG. 9 shows thickness distribution of fibers in accordance with thesecond aspect of the present invention; and

FIG. 10 shows the length/diameter ratio distribution of fibers accordingto the second aspect of the present invention.

The steel fibers of FIGS. 1 and 2 were prepared as described above andmixed with two examples of concrete mix as also described above, and invarious densities (percent by volume) of fiber to concrete, as indicatedin FIGS. 4 to 7. To demonstrate workability, the concrete and fiber mixis poured into an open-ended cone, visible in FIG. 3. When the cone islifted, the slump of the concrete indicates the workability of theconcrete and hence its capacity to flow when pumped or poured into therequisite mould. Depending on the degree of workability required, thedensity of fiber is adjusted accordingly.

With reference to FIGS. 4 to 7, standard concrete blocks are formed andcured and subjected to increasing load while the deflection of thesample is monitored. In FIG. 4, it can be seen that, for normal,unreinforced concrete, load increases with minimal deflection up to amaximum point at which fracture occurs. However, with only 0.16% ofshredded fibers, (in accordance with the second aspect of the presentinvention), substantial deflection of the sample occurs while, stillsupporting a load.

In FIG. 5, 0.64% density of fibers were included in three samples ofnormal concrete in accordance with Example I above. In the first sample,the fibers were in accordance with the second aspect of this invention,namely from shredded tires. In the second sample, the fibers were from acommercially available source (Novocon). The third sample comprisedfibers in the form of strands in accordance with the first aspect of thepresent invention exhibited the greatest loads and defelections, whilethe sample according to the second aspect demonstrated quite acceptableloads.

FIG. 6 demonstrates the substantial loads that are accommodated withhigh strength concrete (according to Example II above) compared withnormal strength concrete (according to Example I above).

FIG. 7 demonstrates the increasing loads capable of accomodation withincreasing density of fiber in accordance with the first aspect of thepresent invention.

Finally, in FIGS. 8 to 10, it can be seen that the distribution offibers employed in the examples according to the second aspect of theinvention have a wide distribution of lenghts and four main thicknesses.This results in a length to diameter distribution in which the vastmajority of the fibers, both in terms of number and volume percent havean l/d ration in excess of 150, and less than 250. The remaining fibers,about 30% in terms of frequency, have l/d ratios between 30 and 150. Asmentioned above, while these will contribute less towards the tensilestrength of concrete, they will add local toughness and wear resistance.

1. A fiber reinforcement structure for concrete, comprising clean steelfiber of between 0.05 and 0.3 mm diameter, wherein said fibers arestranded together in a cord of at least five said fibers, characterizedin that the ends of said fibers in said strand are permanently andstructurally secured together, and whereby the fibers remain securedtogether after dispersion in concrete.
 2. A fiber reinforcementstructure according to claim 1, in which each said fiber has a length todiameter ratio in excess of 150, and said strand has a length todiameter ratio of less than
 60. 3. A fiber reinforcement structureaccording to claim 1, in which said strands have a diameter of 1.5 mmand a length of 50 mm.
 4. A fiber reinforcement structure according toclaim 1, in which there are at least twenty said fibers in each saidstrand.
 5. A fiber reinforcement structure according to claim 4, inwhich said strand comprises an inner core of not less than 10 and notmore than 15 said fibers and an outer sleeve of not less than 12 fibersand not more than 20 said fibers.
 6. A fiber reinforcement structureaccording to claim 4, in which said strand comprises an inner core of 12fibers, and an outer sleeve of 15 fibers.
 7. A fiber reinforcementstructure according to claim 1, in which said fibers have a diameter ofnot less than 0.1 mm and not more than 0.2 mm.
 8. A fiber reinforcementstructure according to claim 1, in which there is less than 100 twistsof said fibers in said strand per meter.
 9. A fiber reinforcementstructure according to claim 1, in which the said ends of said fibers insaid strand are secured together by welding.
 10. A fiber reinforcementstructure according to claim 1, in which said strands are made bycutting to length said wire strands from recycled car and vehicle tires.11. A fiber reinforcement structure according to claim 10, in which saidtires have been subjected to anaerobic heating to strip elastomer fromsaid wire strands.
 12. A fiber reinforcement composition for concretecomprising steel fiber obtained by shredding vehicle tires andphysically separating therefrom non-steel material until “clean” wirefibers remain, about 90% or more of them being individual fibers andsubstantially none having an l/d ratio of more than 250, characterizedin that a majority of the fibers are less than about 0.5 mm in diameter,any wider diameter fibers having an l/d ratio less than
 100. 13. Acomposition according to claim 12, in which a majority of the fibers are0.3 mm or less in diameter and have an l/d ratio between 150 and 250.14. A composition according to claim 12, in which more than 80% of thefibers are 0.3 mm or less in diameter and have an l/d ratio between 150and
 250. 15. A fiber reinforcement structure as claimed in claim 1, inwhich: each said fiber has a length to diameter ratio in excess of 150and said strand has a length to diameter ratio of less than 60, and saidstrands have a diameter of 1.5 mm and a length of 50 mm.
 16. A fiberreinforcement structure as claimed in claim 1, in which: each said fiberhas a length to diameter ratio in excess of 150 and said strand has alength to diameter ratio of less than 60, said strands have a diameterof 1.5 mm and a length of 50 mm, and each said strand contains at leasttwenty said fibers.
 17. A fiber reinforcement structure as claimed inclaim 1, in which: said fiber has a length to diameter ratio in excessof 150 and said strand has a length to diameter ratio of less than 60,said strands have a diameter of 1.5 mm and a length of 50 mm, ach saidstrand contains at least twenty said fibers, and said strand comprisesan inner core of not less than 10 and not more than 15 fibers and anouter sleeve of not less than 12 and not more than 20 fibers.
 18. Afiber reinforcement structure as claimed in claim 1, in which: saidfiber has a length to diameter ratio in excess of 150 and said strand alength to diameter ratio of less than 60, said strands have a diameterof 1.5 mm and a length of 50 mm, each said strand contains at leasttwenty said fibers, and each said strand comprises an inner core of 12fibers, and an outer sleeve of 15 fibers.
 19. A fiber reinforcementstructure as claimed in claim 1, in which: said fiber has a length todiameter ratio in excess of 150, and said strand a length to diameterratio of less than 60, said strands have a diameter of 1.5 mm and alength of 50 mm, each said strand contains at least twenty said fibers,each said strand comprises an inner core of not less than 10 and notmore than 15 fibers, and an outer sleeve of not less than 12 and notmore than 20 fibers, and said fibers have a diameter between 0.1 and 0.2mm.
 20. A fiber reinforcement structure as claimed in claim 1, in which:said fiber has a length to diameter ratio in excess of 150, and saidstrand a length to diameter ratio of less than 60, said strands have adiameter of 1.5 mm and a length of 50 mm, said strand contains at leasttwenty said fibers, each said strand comprises an inner core of 12fibers, and an outer sleeve of 15 fibers, and said fibers have adiameter between 0.1 and 0.2 mm.
 21. A fiber reinforcement structure asclaimed in claim 1, in which: said fiber has a length to diameter ratioin excess of 150, and said strand a length to diameter ratio of lessthan 60, said strands have a diameter of 1.5 mm and a length of 50 mm,each said strand contains at least twenty said fibers, said strandcomprises an inner core of not less than 10 and not more than 15 fibers,an outer sleeve of not less than 12 and not more than 20 fibers, saidfibers have a diameter between 0.1 and 0.2 mm, and there are less than100 twists of said fibers in said strand per meter.
 22. A fiberreinforcement structure as claimed in claim 1, in which: said fiber hasa length to diameter ratio in excess of 150, and said strand a length todiameter ratio of less than 60, said strands have a diameter of 1.5 mmand a length of 50 mm, each said strand contains at least twenty saidfibers, said strand comprises an inner core of 12 fibers, and an outersleeve of 15 fibers, said fibers have a diameter between 0.1 and 0.2 mm,and there are less than 100 twists of said fibers in said strand permeter.
 23. A fiber reinforcement structure as claimed in claim 1, inwhich: said fiber has a length to diameter ratio in excess of 150, andsaid strand a length to diameter ratio of less than 60, said strandshave a diameter of 1.5 mm and a length of 50 mm, each said strandcontains at least twenty said fibers, said strand comprises an innercore of not less than 10 and not more than 15 fibers, an outer sleeve ofnot less than 12 and not more than 20 fibers, said fibers have adiameter between 0.1 and 0.2 mm, there are less than 100 twists of saidfibers in said strand per meter, and said ends of said fibers in saidstrand are secured together by welding.
 24. A fiber reinforcementstructure as claimed in claim 1, in which: said fiber has a length todiameter ratio in excess of 150, and said strand a length to diameterratio of less than 60, said strands have a diameter of 1.5 mm and alength of 50 mm, each said strand contains at least twenty said fibers,said strand comprises an inner core of 12 fibers, and an outer sleeve of15 fibers, said fibers have a diameter between 0.1 and 0.2 mm, there areless than 100 twists of said fibers in said strand per meter, and saidends of said fibers in said strand are secured together by welding. 25.A fiber reinforcement structure as claimed in claim 1, in which: saidfiber has a length to diameter ratio in excess of 150, and said strand alength to diameter ratio of less than 60, said strands have a diameterof 1.5 mm and a length of 50 mm, each said strand contains at leasttwenty said fibers, said strand comprises an inner core of not less than10 and not more than 15 fibers, and an outer sleeve of not less than 12and not more than 20 fibers, said fibers have a diameter between 0.1 and0.2 mm, there are less than 100 twists of said fibers in said strand permeter, said ends of said fibers in said strand are secured together bywelding, and in which said strands are made by cutting to length wirestrands from recycled car and vehicle tires.
 26. A fiber reinforcementstructure as claimed in claim 1, in which: said fiber has a length todiameter ratio in excess of 150, and said strand a length to diameterratio of less than 60, said strands have a diameter of 1.5 mm and alength of 50 mm, each said strand contains at least twenty said fibers,said strand comprises an inner core of 12 fibers, and an outer sleeve of15 fibers, said fibers have a diameter between 0.1 and 0.2 mm, there areless than 100 twists of said fibers in said strand per meter, said endsof said fibers in said strand are secured together by welding, and inwhich said strands are made by cutting to length wire strands fromrecycled car and vehicle tires.
 27. A fiber reinforcement structure asclaimed in claim 1, in which: said fiber has a length to diameter ratioin excess of 150, and said strand a length to diameter ratio of lessthan 60, said strands have a diameter of 1.5 mm and a length of 50 mm,each said strand contains at least twenty said fibers, said strandcomprises an inner core of not less than 10 and not more than 15 fibers,and an outer sleeve of not less than 12 and not more than 20 fibers,said fibers have a diameter between 0.1 and 0.2 mm, there are less than100 twists of said fibers in said strand per meter, said ends of saidfibers in said strand are secured together by welding, and in which saidstrands are made by cutting to length wire strands from recycled car andvehicle tires, and in which said tires have been subject to anaerobicheating to strip elastomer from the wire strands.
 28. A fiberreinforcement structure as claimed in claim 1, in which: said fiber hasa length to diameter ratio in excess of 150, and said strand a length todiameter ratio of less than 60, said strands have a diameter of 1.5 mmand a length of 50 mm, each said strand contains at least twenty saidfibers, said strand comprises an inner core of 12 fibers, and an outersleeve of 15 fibers, said fibers have a diameter between 0.1 and 0.2 mm,there are less than 100 twists of said fibers in said strand per meter,said ends of said fibers in said strand are secured together by welding,and in which said strands are made by cutting to length wire strandsfrom recycled car and vehicle tires, and in which said tires have beensubject to anaerobic heating to strip elastomer from the wire strands.29. A fiber reinforcement structure for concrete, comprising clean steelfiber of between 0.05 and 0.3 mm diameter, wherein said fibers arestranded together in a cord of at least five fibers, characterized inthat the ends of said fibers in said strand are permanently andstructurally secured together, and whereby the fibers remain securedtogether after dispersion in concrete, and in which each fiber has alength to diameter ratio in excess of 150, and said strand a length todiameter ratio of less than 60 and in which said strands have a diameterof 1.5 mm and a length of 50 mm and in which said strands have adiameter of 1.5 mm and a length of 50 mm and in which said strandcomprises an inner core of 12 fibers, and an outer sleeve of 15 fibersand in which said fibers have a diameter between 0.1 and 0.2 mm and inwhich there are less than 100 twists of said fibers in said strand permeter and in which the ends of said fibers in said strand are securedtogether by welding and in which said strands are made by cutting tolength wire strands from recycled car and vehicle tires and in whichsaid tires have been subject to anaerobic heating to strip elastomerfrom the wire strands.